Brushless alternating current (AC) excitation systems are widely used for supplying direct current (DC) field excitation to synchronous dynamoelectric machines such as large AC power generators. Such brushless AC excitation systems include a main exciter having a stationary field structure and a rotating armature member. A diode wheel assembly or rotating rectifier assembly is carried on a common shaft with the rotating armature member and is connected thereto to provide a direct current output. The output of the diode wheel assembly is supplied to field windings of the main generator which also rotates with the rotating armature member and the diode wheel assembly.
A problem of great concern in brushless excitation systems is the detection of rotary circuit malfunctions, such as, for example, ground faults in the field windings of the main generator, shorted diodes or open fuses in diode circuitry of the diode wheel assembly, line-to-line short circuits in the rotating armature member, and phase-to-phase short circuits in the rotating armature member. These types of malfunctions, if undetected, may result in severe and costly damage to the machine and consequential damage to equipment to which the machine delivers power. The effect is particularly serious in the case of a large synchronous generator which supplies critical power to users who may be severely damaged by a sudden power fluctuation or reduction. Therefore, early detection of rotary circuit malfunctions of the brushless excitation system is important, especially since such a machine may continue to operate in apparently normal fashion even after one or more malfunctions occur.
Circuit malfunction detection systems that employ components, such as slip rings for transmitting malfunction information, are not desirably used in operating environments where the use of sparking surfaces should be avoided, such as in a brushless AC excitation system. In a system disclosed by U.S. Pat. No. 3,303,410, the entire disclosure of which is hereby incorporated by reference herein, a ground detection arrangement comprises a pair of rotating shaft mounted collector rings that contact non-rotating brushes during ground fault testing. The ground detection arrangement is metered periodically during operation of the power generating system to monitor for ground faults in the excitation circuit. While this arrangement has been effective for periodically monitoring for ground faults in the excitation circuit, the contact between the brushes and the collector rings may cause sparking and is therefore undesirable. Further, as the system is employed periodically, continuous monitoring for ground faults is not facilitated.
It is desirable to provide alternate means that do not employ the use of slip rings or other sparking surfaces to provide an indication of circuit malfunctions within the rotary electric circuits of the brushless excitation system. One such system employs a strobe light arrangement that is utilized for detecting a failure of fuse members within a diode wheel assembly. However, in this arrangement, a fuse must be located in a position for physical inspection of a fuse indicator, which is actuated when the fuse is opened. As a result, the fuses are mounted on the end surface of the diode wheel. As the synchronous dynamoelectric machines become larger, the number of fuses, or sizes thereof, are increased, which results in an increase of the diameter of the diode wheel, which is undesirable.